1. Field of the Invention
The present invention relates to a line beam generating apparatus, more specifically to a line beam illumination optical system.
2. Background Art
In the conventional art, a line beam can be made in various ways. For example, a truncation type enlarging a beam and truncating the beam into a beam having a higher uniformity than a predetermined value to use as a line beam, a flat-top type uniformalizing a beam by using an aspherical lens or a holographic optical element (HOE) or a diffractive optical element (DOE), a type dividing a beam into a plurality of beams by using adequately arranged optical fibers, a type arranging light sources one-dimensionally and a type of lens array such as a free electron laser (FEL).
The truncation type, however, has a difficulty in improving the uniformity and the efficiency at the same time due to having the trade-off, which refers to losing the efficiency in return for gaining the uniformity or losing the uniformity in return for gaining the efficiency. The flat-top type using the aspherical lens such as a Powell lens has the difficult manufacture of lenses due to the necessity of acquiring a particular distance between each lens and the difficult use due to its sensitivity about the width of an incident beam or a collimation level.
The HOE or DOE has the lower manufacturing performance and efficiency. If emitted beams of light having different emission angles miss their designed paths, the uniformity is lowered. Also, the type using the optical fiber is not suitable for a line illumination having a small Etendue because the Etendue is increased after passing through the optical fiber. Here, the Etendue indicates the width of a light beam as an acquirable important property of a geometric optical system. The width of a light beam is determined by the plane size of a light source and the space angle of a light beam emitted from the light source. The type arranging light sources one-dimensionally and the type of lens array such as the FEL have the optical system more complex and its optical path more lengthened.
If a two-dimensional beam of light incident from a light source is converted to a one-dimensional beam of light in accordance with an embodiment of the present invention, the size (i.e. length) of the illumination optical system is lengthened.